This invention relates to a process for purifying waste water resulting from the manufacture of chemo- and/or thermomechanical pulp, which comprises subjecting said waste water to at least the following sequence of steps:
sedimentation of coarser constituents; PA1 cooling; PA1 detoxification; PA1 anaerobic hydrolysis and preacidification with addition of nutrient salts and readjustment of the redox potential; PA1 production of methane in a methane reactor; PA1 aerobic purification and separation of biological sludge, which is at least partly recycled to the detoxification step.
Such a process is disclosed in European patent application No. 0133846. The known process is destined for waste water resulting from the manufacture of thermomechanical pulp (TMP) or chemomechanical pulp (CMP).
In the known process the hydrolysis step is followed by sedimentation of sludge, which is partly recycled to the hydrolysis step, while the excess of sludge is supplied to the methane reactor. After the methane reactor sedimentation of sludge is effected again, which sludge is partly recycled to the methane reactor, while the excess is discharged. This process has the drawback that the excess of sludge discharged after the hydrolysis reactor, which excess may be considerable, is anaerobic and cannot easily be processed further; moreover, the sludge has an unpleasant smell, which is detrimental to the environment.
It is a further drawback of discharging considerable amounts of sludge after the hydrolysis reactor that said sludge also contains the nutrient salts which are supplied in the hydrolysis step in order to optimize the hydrolysis process and the methane digestion. It is of course advisable that said nutrient salts should as much as possible be utilized in the purifying process and should as less as possible be discharged together with sludge.